The present invention relates to developer compositions for photosensitive elements including lithographic printing plates. More particularly, the invention relates to alkaline developer compositions having etch inhibitors. Even more particularly, the developer compositions have surface active agents which are not good wetters which protect image areas of the lithographic printing plates from etching or attack by the alkaline developer.
The art of lithographic printing depends on several factors which include the immiscibility of greasy ink and water, the preferential retention of a greasy ink by an image area of a printing plate, and a preferential retention of an aqueous dampening fluid by a non-image area. As an aqueous fountain solution moistens the surface of an exposed and developed plate on a printing press, the image area repels the water and the non-image area retains the water. With a subsequent application of greasy ink, the image portion retains the ink and the moistened non-image area repels it. The ink on the image area is then transferred to the surface of the material on which the image is to be reproduced, such as paper, via an offset blanket. The most common type of lithographic plate to which the present invention is directed has a coating of a light sensitive substance that is adherent to an aluminum base sheet. Depending on the nature of the photosensitive coating employed, the plate may be positive or negative working. In both cases, the image area is oleophilic and the non-image areas are hydrophilic. Exposure is effected through a transparent mask or via laser imaging, wherein the light sensitive layer, in the negative working case, hardens and becomes insoluble in a developing solution. When the developer is applied to the plate, the unexposed, non-image areas are removed. In the positive working case, the exposed areas become soluble in an aqueous alkaline developer and are removed thereby. In both cases the remaining areas become the portions which attract greasy ink and are called the image areas. The surface underlying the areas from which the light sensitive coating have been removed are hydrophilic, do not attract greasy ink and are called the non-image areas.
Generally, these plates are composed of an aluminum containing substrate which may or may not have been treated with a variety of processes recognized in the art including anodization, graining and hydrophilization. The prepared plate substrate may then be applied with a photosensitive coating comprising a photosensitizer, binding resins, colorants, acid stabilizers, surfactants and other art recognized components. Common photosensitizers include diazo compounds, including polymeric diazonium condensates salts and o-quinone diazides, and photopolymerizable compositions. Sensitizers, binders and printing plates employing aromatic diazonium compounds are described in U.S. Pat. Nos. 3,175,906; 3,046,118; 2,063,631; 2,667,415; 3,867,147 and 3,679,419.
The photosensitive layer may be exposed to suitable radiation either through a transparency or directly using laser imaging techniques. Hence, development of the plate with an aqueous alkali solution should result in the exclusive removal of the non-image areas of the photosensitive layer.
A number of positive acting printing plates are supplied commercially, which include the P-51 manufactured by the Agfa Division of Bayer Corporation.
The invention also relates to developer compositions for negative working photographic elements, such as lithographic printing plates. These include the N-81 and N-90, products of Agfa Division of Bayer Corporation.
The present invention is also useful for developing color proofing films. Such color proofing films assist a printer in correcting a set of photomasks which will be used in exposing printing plates. Visual examination of a color proof should reveal any defects on the photomask, the best color rendition to be expected from press printing of the material; the correct gradation of all colors; and whether there is a need for changing any of the colors and/or for altering the film photomask before making printing plates.
Generally, color proofing films are composed of a transparent substrate, which may or may not have been treated for release, slip or adhesive properties. The transparent substrate is then applied with a photosensitive coating comprising a photosensitizer, binding resins, colorants, acid stabilizers, surfactants and other art recognized components. Common photosensitizers include o-quinone diazides, diazonium condensates salts and photopolymerizable compositions. They are exposed and developed in a similar manner as printing plates. U.S. Pat. Nos. 4,260,673; 4,093,464; 4,093,464; 4,260,673; and 4,659,642 teach typical color proofing systems.
It has been found that during the development of positive and negative working photosensitive elements, there is a tendency for the developer to also attack the image areas of the exposed photosensitive coating. This results in an unwanted etching away and removal of image areas of the element, thus producing a defective image. The present invention solves this problem by the inclusion of certain etch inhibiting compounds in the developer composition.
The developer composition inhibits etching of the image areas of the lithographic printing plate. The etch inhibiting additive allows use of the developer in concentrations and under temperatures that would otherwise cause degradation of the image areas on the plate. This provides greater operating latitude for exposure requirements, such as time and energy, mechanical agitation and developer contact time to produce printing plates of suitable quality. Developers of this invention are suitable for use with an array of printing plates, including lithographic printing plates, color proofing films, positive and negative working lithographic plates and thermally sensitive plates.